Automatic ganglia are studied here as a relatively simple model of synaptic interactions in the vertebrate nervous system. The investigation focusses on the nature of the slow postsynaptic responses, lasting seconds to minutes, including the electrogenic processes and chemical synaptic transmitter pathways. We have established that dopamine is a direct transmitter for the slow inhibitory postsynaptic response, but that it also acts to produce a long-lasting enhancement (several hours) of the slow depolarizing (excitatory) response to another transmitter, acetylcholine. One chief avenue of investigation will involve the nature of the enduring neuronal changes induced by dopamine, which are the basis for dopamine-modulation of the postsynaptic response to acetylcholine; this includes the roles of the cyclic nucleotides in mediating such modulation. Other avenues include analysis of the postsynaptic receptors that may be involved in the different actions of dopamine.